The Silver Surfer was cast away by Galactus before, in his original story he had betrayed Galactus so Galactus left him in a diminished state and trapped on Earth behind an invisible cosmic barrier. Surfer then became part of a comic called the Defenders with Strange, Namor and Hulk. Finally he broke out of Earth stories and took part in arcs like the Infinity Gauntlet and Annihilation Wave

Was Surfer's strength lower and speed lower when he was trapped on Earth?

In his first series of 18 comics from 1968-1970, other than being trapped (by Galactus' invisible barrier) in a world (Earth) that only saw him as a menace (for the most part), he had all his powers, speed, etc.

As far as speed goes, in one of the books (Silver Surfer #6), he travels through time (using his speed) in an attempt to free himself.

As far as power is concerned, in just the first six books of this series, he defeats (or at least fends off) many formidable foes: The Badoon, Mephisto, Thor, The Stranger, The Overlord, etc.

Being trapped on Earth and tormented by the loss of Shalla Bal (only to be "teased" by her unexpected cameos as manifested by his enemies) was his greatest weakness. Galactus really knew what he was doing in the punishment department!

Just my $0.02

I never got that indication. But he certainly received a power upgrade in Annihilation (see Beta Ray Bill: Godhunter #2).

Galactus removed his 'space time' powers when he exiled him, although that didn't seem to stop the surfer travelling in time and to other dimensions. Presumably it was just to stop him warping past the barrier. What is often forgotten is that the Surfer had his powers reduced to' a fraction ' by the sonic shark weapon that SHIELD used on him. In the first run of his own comic the Surfer was worried by falling comparatively small heights and remarked to a foe 'Once I might have felled you with a shrug, but though I am less than before, I am still the Silver Surfer' He did seem to vary in power in various stories with some writers seeming to forget that he had been de-powered. Most online guides to the character state that he slowly returned to his full power.

The first Marvel Universe Handbook, stated that the Surfer always passively absorbs cosmic energy even though this is slower on a planet than in deep space and that he can increase the rate of absorbtion. He can also channel cosmic energy, so it's not like he is just some recharging battery That makes any story involving the draining of his power a bit suspect to me. To truly steal his power as Doom did you would have to not only drain his existing store of energy, but also duplicate his ability to channel energy from outside and stop the Surfer from doing the same. The early writers didn't have to worry about this as no-one gave much thought to how powers worked until the Marvel Universe Handbook was first published. However, there are still recent stories where the Surfer's power is stolen, such as when the Bllack Panther did it. Given how the Surfers powers work, he should be able to rapidly recharge (particularly in space) and in any case could still channel outside energy. It's just lazy writing or lack of knowledge about how the Surfer's powers work in my opinion!