LVM snapshot merge is slow and somethims gets stuck

I am having some issues related to lvm snapshot merge.

When running

lvconvert --merge <snasphot>

on ubuntu 14.04.6 (kernel 3.19.0-80-generic) everything worked fine.
Lvm version is:

  LVM version:     2.02.98(2) (2012-10-15)
  Library version: 1.02.77 (2012-10-15)
  Driver version:  4.29.0

Running the same command on ubuntu 18.04.4 (kernel 4.15.0-76-generic) is much slower (can take few hours).
Lvm version is:

 LVM version:     2.02.176(2) (2017-11-03)
 Library version: 1.02.145 (2017-11-03)
 Driver version:  4.37.0

Also,
there are sometimes when the merge gets stuck, when looking on

dmestup status <lvm-base> 

I can see that the merge is stuck.
The output is

0 8388608 snapshot-merge 16/2097152 16

and there is no change, no progress and the snapshot is not merged.

Any ideas what could be the problem?