MAME's -listfull command now includes the device sets, which it didn't before 0.195. QMC2 uses the -listfull command to determine the number of supported sets in a very early stage (because it's fast and because QMC2 needs some numbers to make the progress bars somewhat meaningful - *). Since QMC2 also uses the number to compare it to the number of sets in some of its caches, older versions will *think* the caches are out of date and clear them. The result is that startup times are ~10x higher than needed.
It's actually good that -listfull includes device sets now, but it required some changes to QMC2's startup logic. The good news for QMC2 is that it will now detect the need for a cache refresh even when only device-sets have changed / were added. And, since the number of supported sets was actually just the number of non-device sets in the past, not all progress bar ranges were 100% correct (only in case of parsing the XML initially, though). That's now auto-fixed as well
*: This number is also "cached" (stored in qmc2.ini), and QMC2 detects any such change by comparing the SHA1 hashes of the -listfull output and the one from the last cache refresh (also stored in qmc2.ini).