After upgrading to Lubuntu 20.04 launching chrome takes a long time

Lubuntu 18.04 LTS was the last Lubuntu release to use LXDE.

Release notes from 18.10 up provided the warning, eg. on Lubuntu 20.04.2 release notes say clearly

Note, due to the extensive changes required for the shift in desktop environments, the Lubuntu team does not support upgrading from 18.04 or below to any greater release. Doing so will result in a broken system. If you are on 18.04 or below and would like to upgrade, please do a fresh install

For the 18.04 to 18.10 upgrade; some notes existed for users that didn't want to clean install, however they weren't fully published and no longer exist (may exist in git somewhere rather than a readily readable format).

If you clean installed Lubuntu 18.10 (the first LXQt release), upgrades were possible from then on again normally.

The problems were not the same for all users, as complexity of problems varied on what packages were installed, and many issues were cosmetic in nature (ie. you're talking about slowness, rather than not running), but some items (I'm thinking menu items that went nowhere) were more of an issue/distraction.

The warnings on upgrades were there for a reason.

I'd recommend reading the release notes in future. The upgrade you performed was unsupported for a reason; problems were to be expected.