My recommendation would be to just stop tracking XP.I guess that could work. I've had bad experiences with minions in the past - too much XP for too little risk - but they would run by much faster. (That was mostly my fault though, inexperience.)
The thing is, I want to maintain the challenge, but get along to other things as well.
Do you recommend dropping monsters and levelling up the others? That is, keeping the XP budget the same?
Just an update, in case it had any interest.
Last night, in my usual campaign (players are level 17), I changed all monsters (so easy with Monster Builder) by removing 33% of their hit points and adding +50% damage (for example, 3d8+7 became 3d8+17).
I was very happy with the result. The fights were fast and furious. The first 3 fights were fairly simple n to n+2 fights. All three lasted between 15 and 24 minutes, including initiative setup etc. The last fight lasted 1h15 but that was *final battle* against a trio of vampire - so lots of regeneration and dominating effects. It was supposed to be long, but I didn't expect (after the duration of the 3 first fights) that it would last that long. Should probably have clocked in around 45 minutes, but I made the list of powers of the vampires way too long and I think I took too much time per round.
Either way, I am still pumped about last night. 15-25 minutes is perfect (for me) for fast and brutal fights, but with a certain amount of tactical options.
How fast did the players use up their healing surges for each combat encounter?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.