What rule system did you use for your previous campaign? I'm going to take a wild guess and say it was 3.5.
In 3.5 it was also easy to create monsters because each monster manual had the template already done for you.
Changing things around on the various monsters didn't take very long.
If your "monster" was an evil cleric (or wizard) you would need to do the following:
1) Create the cleric, pick feats, skills & domains (or specialty school). Decide on any template. (Remember, in 3.5E, there was no official software that allowed you to create characters - Heroforge was good, but not perfect)
2) equip the cleric or wizard with items appropriate for their level, including magic items.
3) pick many dozens of spells for the evil cleric or wizard once the game gets to a high enough level. Decide which spells are meta-magicked and slot them appropriately in the process. Does this wizard or cleric have a contingency item and/or spell on them?
4) Make the evil cleric or wizard different than the previous cleric or wizard so the encounter has a unique flavor to it. This was very important to me as a DM - I didn't want to do the same thing over & over again, just adding levels along the way.
5) Since I had a large group of players (8), I could not throw a solo cleric or wizard at them - they would need to have support, be it warriors, monsters, summoned creatures, lower level clerics/wizards or whatnot. So, if the level 15 cleric had a level 12 and a level 11 with him, I would need to do the same with those supporting clerics. Then, the clerics would need bodyguards - fighters or monsters of whatnot.
6) Forgot this part - but, then you need to decide which "buffing" spells/items the bad guy is going to cast and in which order (i.e, what if the PCs surprise the bad guys, what if the bad guys surprise the PC, what if they stumble upon each other, what if the bad guy has only one round of warning or two...) and, then you need to calculate the plusses and/or minuses for each buff. Individually, the calcs are all easy, but if you have 6-8 buffing spells and some stack with others and some don't, it can get confusing if certain ones are dispelled and others are not)
Then, I need to decide if the PCs will have any allies to help them out at all, too.
And, since my bad guys had dozens of different options each round, I would often be a bottleneck at the table deciding which spell to cast, which metamagic spell, where to move, etc for each of my 15-20 bad guys. And, after each encounter, I would kick myself because I missed one or more important options for my bad guys ("damn, my lich had a ring of positive energy resistance on her left hand..." I totally forgot about it since most of her other items were protective in nature, like the Ring of Protection +4 on her other hand. But, the party rogue doing 40 points of positive energy damage to her was a turning point in the battle. )
With 4E, I just decide on the level of the encounters and pick appropriate bad guys from the monster builder, copy & print. They're already equipped and have their skills all picked out and I don't need to pick out 40 or more spells and 5-10 magic items. Where I was spending 20-40 hours between sessions on creating bad guys and encounters in 3.5E, now I'm spending 20-40 minutes.
While I like Pathfinder, I found it made the base classes more complex than 3.5E, so it would have even more things for me to forget/not remember. Plus, when my group chose 4E over Pathfinder a year and a half ago, there was no official character building software available for PF at the time.