Cheap, interactive spells are the best way to gain tempo in Magic. Having those spells be instants is how you get even further ahead in the tempo game. Being able to leave up a few mana to represent so many different options during the opponent’s turn makes a deck extremely hard to battle back against.
The card that makes all of this happen in Modern is Snapcaster Mage. It ambushes creatures and attacks for 2, but mostly provides incredible value. In conjunction with any other cheap spells, Snapcaster Mage represents at least a 2-for-1. Sometimes the flashbacked spell offers its own 2-for-1, making the value such an incredibly high amount that scientists would be unable to calculate how far ahead you are.
Burn spells are another great way to generate tempo. Lightning Bolt is already such an efficient removal spell, but Bolt, Snap-Bolt alone represents 6 damage, and then a clock of 2 more per turn. In conjunction with more Bolts, more Snapcasters, and then maybe some Lightning Helixes and Electrolyzes, you’re looking at a ton of damage. Keeping creatures off the board to keep attacking or going straight for the face are both effective.
Do I need to say why Path to Exile is good? People don’t play many basics, so while it’s not a great turn-1 play, it trades efficiently later by removing a large threat for a single mana.
Remand is similar to Time Walk in a number of matchups. In others, you’ll want Spell Snares and Mana Leaks to keep things under control. In the early game, you don’t really want to cast Path to Exile and you want to hold your burn spells as long as possible. A Snare or Leak snagging Dark Confidant or Tarmogoyf will keep you ahead early.
If you want to maximize your tempo, you can counter their early plays and flash back counters with Snapcaster Mage to start attacking quickly. Spell Queller does all of that in just a single card, however. Many decks in Modern play zero cards that can’t be hit by Queller. It’s just too hard to play expensive spells in a format where Remand is popular. Spell Queller will make sure that the game stays in your favor before attacking as an evasive clock. Setting up a turn-2 counter into turn-3 Queller means that you have all your mana available for Snapcaster Mage and other interaction on turn 4.
Your early turns are going to be spent setting up your future tempo plays. Serum Visions is a prominent player in Modern as a way to dig deeper into your deck and make sure your draws stay consistent. Ancestral Vision is a powerhouse that was worthy of being banned for much of its existence. Having an extra mana laying around to suspend it is generally trivial, but the 3 cards you draw are anything but. With so many cheap spells to cast, you should be able to empty your hand quickly, and then empty it again after refilling.
With all of the burn spells and ways to flash them back, the game tends to end quickly. Throw in Spell Queller as well as Vendilion Clique for some efficient flying clocks that will greatly disrupt your opponent, and you have another consistent angle of attack. Spell Queller continues to show up in larger numbers in Modern and may be even more powerful in older formats than it is in Standard!