Boris Johnson warns Russia has “60 … battle groups” (presumably battalion tactical groups) ready “for a lightning war that could take out Kiev”.
He also said that if Moscow sent troops in, it would be a “painful, violent and bloody business – and I think it’s very important that people in Russia understand that this could be a new Chechnya.”
If forced, Johnson said he believed the Ukrainians would fight, and he said the Kremlin should understand that.
https://www.theguardian.com/world/2022/jan/24/johnson-warns-of-painful-and-violent-ukraine-lightning-war
Johnson was warning that Russia could get bogged down. In fact a US military analysis of the 2014 Russo-Ukranian conflict concludes the Kremlin’s battle groups fared poorly when they faced with multiple and dispersed threats.
To defeat a BTG, increase uncertainty and shape the battlefield by “burning more calories” to overload the BTG commander’s most valuable systems and personnel. Once hostilities are initiated, attack on multiple fronts to destroy his maneuver force, displace his mission command, EW and fires assets, and seize his sustainment area.
https://www.benning.army.mil/armor/earmor/content/issues/2017/spring/2Fiore17.pdf
The battle groups are designed for short, rapid tempo operations, not prolonged slogs.
The second issue was that the Russian Army had too few contract soldiers to man the current and future force structure. High casualties in Chechnya and Georgia significantly depressed volunteer recruitment. Russian military leadership wanted to avoid a similar situation where high casualties in Ukraine might further depress recruitment. As a result, even though the BTG represents the best personnel a Russian brigade can deploy, two-thirds of the deployed personnel are unsuitable for close combat, and the third that is combat-ready is too valuable to risk unnecessarily.
https://www.benning.army.mil/armor/earmor/content/issues/2017/spring/2Fiore17.pdf
So for a variety of reasons Russia must avoid a long, indecisive campaign.