you could stop the plugin, delete the log, and start it again so its shorter. you could also search the log for only errors and warnings not the usual debug stuff.
what do you mean by loading the plugins slow?
if you mean the plugins > installed tab, that may mean one/some of your plugin jail(s) has unreliable networking.
for the abpve case, I recommend disabling VIMAGE and assigning an interface to your plugin jails. this usually makes networking for them much more reliable.