If you want to become big again after splitting your cell quicker than waiting for them to absorb each other, why not try finding a wall and corner to eject mass towards for one of your cells to eat. This is a strategy used by many players. you can do this without a wall when in 3+ pieces, just feed the piece you want to feed but aiming at one of your small pieces.
Even though a bunch of small cells may seem like an easy target, be careful, because it can merge into a cell bigger than you (especially if there is one big cell). If there is more than 6 or 7, do not try and eat that player unless if you can split and eat them.
Use viruses to your advantage by hiding behind them (not in them if you are big) to block another player from eating you. They don't want to split, so they will not pursue you, just make sure that you do not split.
If a big cell is chasing you, find a virus and make them think you would shoot them and they will leave you alone.
When getting chased by a bigger cell that wants to split-kill you, try moving up and down to dodge their split to make it harder for them to catch you.
If you have 400 mass or more, you can eat viruses (though not in Teams Mode). Make sure you are in a place where there are no other cells and many viruses. Split on 1 of the viruses, make sure you are in 16 pieces first, and then use your biggest piece to scoop up the rest of the viruses.
You can split to shoot a virus at someone. they most likely won't see you coming, and if you have a small piece and someone overlaps you, lead your small piece to a virus and shoot the overlapping enemy. make sure you don't shoot yourself by being a little farther to the virus than your enemy.
If someone overlaps you and you're in 2+ pieces, stay on top of them until you merge. BUT, be careful not to merge a piece that is too small right ON your opponent, instead, use your big piece and watch their movements, then the small piece will gradually come near and merge. this can kill big opponents and especially nasty teamers.
if someone viruses you, use your small pieces to bait other players to you and kill them. Alternatively, eat some viruses for more mass, but only if you are still in 16 pieces.
Play aggressively when you're big (10k+) but don't get too aggressive; hitting 2-3 viruses and self-feeding can activate the Anti-Team Penalty, which can cause you to lose over half of your mass in a few minutes. Knowing how aggressive to play and how to avoid Anti-Team Penalty is a skill in itself.
It's generally a good idea to eject one mass into a cell (player) much larger than you. They won't bother eating you If you're too small and since you gave them mass they will either team with you or won't attack you. Ejecting to a skilled player and avoid dangers, but watch out for solo players as they generally think that mass shooting means that you're a teamer.
When you are sure that you are teaming with another player let them eat your smaller cell if you somehow split from a virus, and eat their smaller cell if they split from it. However, try to stay in 16 pieces if you are facing a virus barrage, as you will only get drained by Anti-Team Penalty.
You can't always trust the players with names like "u help=i help" and "team?", especially if they already have a teammate.
Be careful when teaming in Party. when you split, your teammate might betray you and eat all of you, resulting in a game over. These cells are called Fake Teamers..
If using Windows, try turning off the mouse precision setting. Control panel, mouse, mouse properties tab, un-check the box for "Enhance pointer precision". If checked, that setting seems to make the cursor want to stick to your cell. The cell is more maneuverable if that box is not checked. In that same spot, you can set your mouse speed to fast, and little mouse movements will have you zooming around.
If using a PC, make the setting adjustments suggested above, then set your browser to full screen—more space to see danger and the browser seems more responsive.
If you have a wide screen monitor, travel your cell left and right, not up and down. That gives you lots more time to see threats.
If using a PC, the Agario start screen has a resolution setting. For me, there is no difference in the cell appearance if I select low resolution, and the cell is more responsive.