Children develop at slightly different speeds in their bilingual language development. Just as some children learn to crawl and walk earlier than others, and say their first words earlier than others, so the speed of language development varies between children. This is even more so in the development of two languages.
The speed of language acquisition is only partly due to the child's ability. Indeed, some children who turn out to be very able in academic terms are slow in their language development. There is generally little relationship between how quickly someone learns to speak one or two languages and eventual school success. Early language developers are not likely to be more successful in adult life - however success is defined.
A child's interest in language is also important and is partly separate from ability and aptitude for language learning. When a child is encouraged and stimulated in language development, an interest in reading, for example, will be increased. Parents, who listen with attention to what the child is saying, answer the child in a child-centred way, make language fun by rhymes and songs, will tend to aid language development. A child's interest in language and motivation to engage in lots of conversation will affect their speed of development.
Given adequate encouragement, practice and a stimulating environment for language growth, children find the acquisition of two languages relatively straightforward, painless and effortless. Many children tend to reflect parents' attitudes, behaviour, expectations and beliefs. A positive parent tends to breed a successful child. Parents who expect failure tend to breed less success. This is particularly true of a child's bilingual development.
There are many hurdles but few insurmountable barriers to children reaching whatever bilingual language destination is possible. However, routes to dual language proficiency are long. This is one reason for undue concern by destination-seeking parents. Early acquisition of two languages for some parents seems like slow motoring. For other parents, the journey in later childhood and the teenage years seems troublesome, even like back-pedalling. Such concerns seem usual and prevalent among parents of bilingual children. However, if safety in numbers doesn't satisfy, consider comparing your bilingual child against other bilinguals and not against the fastest moving monolingual who sets the pace. Many bilinguals show similar language performance to monolinguals in one of their languages. Some, but not all, develop considerable competence in a second language. Rarely are bilinguals equally fluent in all situations in both their languages.
Not all bilingual children will reach the same language destination. Family, community and education circumstances sometimes mean the journey halts at passive bilingualism (that is, understanding but not speaking a second language). Partial or passive bilingualism is not a finishing line. Given a need to become an active bilingual (e.g. by visiting the country where the hitherto ‘passive' language is dominant), the journey can be continued to more complete bilingualism.