So it’s kind of like using 5 characters from a much larger alphabet (the English dictionary) instead of 30 from a 26 letter alphabet.
Even on the low end, it should take well over a decade if LastPass chose a good cryptographic hash function with a high iteration count.
The problem is that no average person is gonna use a password that long to begin with.
Most people know at least 3 languages well enough. I often make passphrases out of 5 to 6 languages.
Diceware - an actual formalisation of the approach, including recommended means of generating the entropy and specific wordlists - uses 7776 words, for a shade under 13 bits each. EFF have a nice one - https://www.eff.org/dice
The recommended 6 words gives 77.5 bits of entropy per password. At Lastpass's current default iterations of 100k that's about 2^93 SHA256 operations to have better than even odds of breaking it.
To put that into perspective, the Bitcoin mining network is reportedly hashing at 256 quintillion hashes per second right now. At that rate it would take on average 72 weeks to crack.
One extra word bumps that up to 11 millennia, if that's a bit too tractable for comfort.
It is that it is a method for which the large group of people who would choose "Password0!" as their password [1] can use to create a password they can potentially remember, but such that the chosen password is "relatively" more secure than the basic alternative they would have otherwise chosen.
[1] I.e. the far too common method of "pick a word, make one or two letters capitol, append a numeral, append a !".