.. it becomes a nightmare really fast if you want to build complex battle scenario
If your intention is to build a complex battle scenario then just accept that complexity is complex and it is slow. That is what you wanted and that is what you will get.
If you want fast then you need to simplify.
We had a GM, that like you, individualised every combatant and rolled all their rolls. When we saw a big group we would inwardly groan as we know we would probably get one attack an hour. The first reaction after rolling initiative was for most of the party to get up and go to the kitchen to make a cup of tea. I would suggest you look at these big fights and ask yourself what purpose do they serve? Are they really life or death encounters? Are you prepared to kill a PC during this fight? Is it just to weaken the party to make the upcoming climax more challenging? If it really is a life or death fight then I can see the point of playing things by the book. If it is simply incidental then is this the best use of the limited time you and your players have each session?
I am at the totally different end of the spectrum to you. I tend not to even define how many combatants there are so I can be elastic with the level of challenge. My fights tend to be more like Hollywood fights where despite there being a castle full of defenders they come at the party members in pairs and singles. They may fight a guard as they entered a castle tower, another on the stairs, two on the wall. I don't know how many there are and nor do I care. There are enough guards to make it challenging and as they come in small waves I can have fun with them such as having two guards pick up a chaise longue and charge the party with it, another may push his two mates forward and then flee. If I wanted a grande melee then really only the 10' radius around the characters matters, beyond that the actual outcome of the battle is more likely a story aspect than down to an open-ended roll surely?
If you really want to do this then I would suggest to do as much as possible in your prep time and not at the gaming table. You could produce small flow charts of what each combatant will do in the first round then branch it so you know what they will do in the second round if they are uninjured or if they are slightly wounded and then severely wounded. Put each combatants flow chart on an index card and use that to also record their OB/DB/Hits so it serves as your entire combat record. Once initiative is rolled sort the cards into the right order. I would even go so far as to roll a dozen D100 rolls and write them across the top of the card so you can skip the dice rolling. That will save five/ten seconds per combatant per round as you try and find your dice, roll them and assimilate the result. I would quite literally put as much information on that card as you can even down to if they have any spells then jot down the spell definition so you do not need to refer to SL.