Desktop is too slow, current state and some benchmarks (1/x)

So my desktop computer is becoming too slow for even the lightest photo editing tasks.  Don't even try video.  Here's the current setup:

$ cat /proc/cpuinfo
processor    : 0
vendor_id    : GenuineIntel
cpu family    : 6
model        : 15
model name    : Intel(R) Pentium(R) Dual  CPU  E2200  @ 2.20GHz
stepping    : 13
microcode    : 0xa4
cpu MHz        : 1290.420
cache size    : 1024 KB
physical id    : 0
siblings    : 2
core id        : 0
cpu cores    : 2
apicid        : 0
initial apicid    : 0
fpu        : yes
fpu_exception    : yes
cpuid level    : 10
wp        : yes
flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl cpuid aperfmperf pni dtes64 monitor ds_cpl est tm2 ssse3 cx16 xtpr pdcm lahf_lm pti dtherm
bugs        : cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf
bogomips    : 4399.87

Memory is ok (nobody was logged in):

$ cat /proc/meminfo | head -20
MemTotal:        4039364 kB
MemFree:         2716668 kB
MemAvailable:    3529372 kB
Buffers:           70428 kB
Cached:           899376 kB
SwapCached:            0 kB
Active:           612904 kB
Inactive:         482644 kB
Active(anon):     126648 kB
Inactive(anon):    12484 kB
Active(file):     486256 kB
Inactive(file):   470160 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       4085756 kB
SwapFree:        4085756 kB
Dirty:              5460 kB

I'm testing the system disk performance with bonnie++ (this is new to me):

$ time bonnie++ -d /tmp -s 0 -n 100 -f
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version  1.97       ------Sequential Create------ --------Random Create--------
kid3                -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                100 34604  72 +++++ +++ 45834  69 36578  74 +++++ +++ 40212  63
Latency             23276us    1109us    1045us     366us      55us     136ms
1.97,1.97,kid3,1,1539800153,,,,,,,,,,,,,,,100,,,,,34604,72,+++++,+++,45834,69,36578,74,+++++,+++,40212,63,,,,,,,23276us,1109us,1045us,366us,55us,136ms

real    0m11,145s
user    0m0,483s
sys    0m7,505s

For reference, the same command on the local system disk on my (about 2x faster) laptop:

$ time bonnie++ -d /tmp -s 0 -n 100 -f
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version  1.97       ------Sequential Create------ --------Random Create--------
run12l              -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                100 60110  55 +++++ +++ 85174  60 68694  63 +++++ +++ 80560  59
Latency               283us     525us    4572us    1961us      46us    5125us
1.97,1.97,run12l,1,1539794180,,,,,,,,,,,,,,,100,,,,,60110,55,+++++,+++,85174,60,68694,63,+++++,+++,80560,59,,,,,,,283us,525us,4572us,1961us,46us,5125us

real    0m6,009s
user    0m0,200s
sys    0m3,520s

Same command again but access to the desktop system disk through NFS from my laptop.  I used much smaller number of files because it's a lot slower that way:

$ time bonnie++ -d /net/kid3.lan/home/luc/tmp -s 0 -n 5 -f
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version  1.97       ------Sequential Create------ --------Random Create--------
run12l              -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                  5   569  10 +++++ +++  1595  13   571  10  3440  19  1562  13
Latency             20565us   36590us    1070us   10365us    1543us    3710us
1.97,1.97,run12l,1,1539795060,,,,,,,,,,,,,,,5,,,,,569,10,+++++,+++,1595,13,571,10,3440,19,1562,13,,,,,,,20565us,36590us,1070us,10365us,1543us,3710us

real    0m26,198s
user    0m0,098s
sys    0m2,987s

Boththe laptop and the desktop are directly linked through an old ethernet hub/router/switch D-LINK DSL-604T that does not support higher than 100Mbps):

run12l $ ethtool eno1|grep Speed
    Speed: 100Mb/s

kid3:~$ ethtool enp0s15|grep Speed
    Speed: 100Mb/s

Partitions/Disks on the desktop, sdb is the system disk and sda1 is a larger disk holding all the non-system data:

$ blkid
/dev/sda1: LABEL="HOME" UUID="501fb987-b347-4b94-a2ac-97d971e28664" UUID_SUB="42cc63c7-590c-4aaf-b850-9e797ed2969e" TYPE="btrfs" PARTUUID="859ce7cb-01"
/dev/sdb1: LABEL="PQSERVICE" UUID="04D2821BE49BED2A" TYPE="ntfs" PARTUUID="2bd2c32a-01"
/dev/sdb2: LABEL="ACER" UUID="E0D8E5D0D8E5A4CE" TYPE="ntfs" PARTUUID="2bd2c32a-02"
/dev/sdb3: LABEL="VIDEOS" UUID="A46020DA6020B4C8" TYPE="ntfs" PARTUUID="2bd2c32a-03"
/dev/sdb5: UUID="b140a7c9-071d-48e2-861e-44242b16fce0" TYPE="swap" PARTUUID="2bd2c32a-05"
/dev/sdb6: UUID="41402262-1350-461f-bed3-b0e8e73610b7" TYPE="ext4" PARTUUID="2bd2c32a-06"

$ mount|grep sd
/dev/sdb6 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
/dev/sdb2 on /windows type fuseblk (ro,relatime,user_id=0,group_id=0,allow_other,blksize=4096)
/dev/sdb3 on /videos type fuseblk (rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096)
/dev/sda1 on /home type btrfs (rw,relatime,space_cache,subvolid=5,subvol=/)