I disagree partly here - throwing waves of deadly encounters is not the way to go about it, unless your group are happy with TPKs.
Players are supposed to defeat a medium or hard encounter. Theyre expected to win, expending a few resources. In fact theyre expected to win 6-8 of these medium to hard encounters in a row (with only a few short rests) before being drained of resources to the point that they need to long rest.
If your PCs are smashing your encounters, just throw more at them; dont ramp up the difficulty.
Also, CR is a guideline only. 5 x 5th level PCs have an encounter budget for a 'hard' encounter of 3750-5499xp. A single CR 5 monster is worth 1800xp - making it an easy encounter for a party of 5 PCs'. A 'solo' CR 9 encounter is 5000xp meaning (on XP alone) it is roughly an appropriate encounter for 5 x 5th level PCs.
You only use the CR as a guide to assess that the monster may have attacks or abilities that could prove lethal to lower level PCs. Taking the CR 9 example, it includes things like a Young Blue Dragon (10d10 damage breath weapon would wipe out a party of 5th level PCs) or a Fire Giants +11 to hit, 6D6+7 damage (multiattack) greatsword is equally deadly. Such an encounter would likely result in a TPK.
Its tempting to increase the difficulty and lower the number of encounters, but dont do it. Taking 5E's swingy nature into account, you'll only kick off an arms race and wind up with TPKs.
Just use more encounters, not harder ones.