When I open a folder that has subfolders in it with lots of files Thunar freezes for some time (up to 30 seconds) and HDD activity grows to 100% read.
I have Show thumbnails option turned off so it's not about thumbnailing.

iotop says that lots of Thunar processes get spawned while opening a folder:

  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND 
 5633 be/4 bornmw    158.03 K/s    0.00 B/s  0.00 % 62.30 % Thunar --daemon
 5600 be/4 bornmw    105.36 K/s    0.00 B/s  0.00 % 55.01 % Thunar --daemon
 5640 be/4 bornmw     90.31 K/s    0.00 B/s  0.00 % 35.61 % Thunar --daemon
 5597 be/4 bornmw     48.92 K/s    0.00 B/s  0.00 %  9.24 % Thunar --daemon
 5634 be/4 bornmw     18.81 K/s    0.00 B/s  0.00 %  6.72 % Thunar --daemon
 5601 be/4 bornmw     11.29 K/s    0.00 B/s  0.00 %  6.27 % Thunar --daemon
 5599 be/4 bornmw     30.10 K/s    0.00 B/s  0.00 %  3.22 % Thunar --daemon
 5598 be/4 bornmw     52.68 K/s    0.00 B/s  0.00 %  2.64 % Thunar --daemon