Logically yes it would make it longer to level the higher the level is, so the extra levels you gain by the enchantments will take you longer to level up in that skill, but on the plus side you get better potions due to the higher skill.
Its all about what you are using alchemy for, if it is for the potions then keep the enchanted items, but if it for the skill level ups then get rid of them and you will level alchemy quicker.
But if it is for overall character leveling, then I would keep them, as the higher the skill you level, the more the overall character level bar fills.
For example a level up in alchemy from 20 to 21 will fill your overall level bar by 10pts, but a level up from 40 to 41 will fill it by 30pts so your charater levels faster the higher the skills you level.
Actually NO it will not make it longer to level at all.
Each skill you use applys a usage point.
So lets define terms.
SKILL_LEVEL = my skill level in this skill type
MODDED_LEVEL = my skill level after it has been buffed up by some item
per_usage_skill_point = the amount of usage points I have been given by using a skill
So to explain this clearly, all skills have an amount of points that must be obtained to increase your "Level" of the skill.
so lets say for skill
you need 1000 per_usage_skill_points to get to a SKILL_LEVEL of 100.
However it is not as simple as dividing 1000 usage points by 100 skill levels to say you need 10 per_usage_skill_points per level. it is skewed so at lvl 1 you would need like 3 per_usage_skill_points to level to SKILL_LEVEL 2, while at SKILL_LEVEL 99 you would need something like 30 per_usage_skill_points.
Anyway, the MODDED_LEVEL only changes the spells you have access to. So for instance, if you cast that lvl 1 spell, lets say you get 1 per_usage_skill_points per cast. So it would take like 3 casts to get to level 2. BUT if your SKILL_LEVEL is only 1, you do not have access to higher level spells. BUT if you get an item that mods your level by 100, you get access to higher level spells that would give you greater per_usage_skill_points. So if you cast a master level spell that gives like 12 per usage skill points, you would instantly raise to lets say SKILL_LEVEL 3.
You see?